1. Field of the Invention
The present invention is related to a communication system, a communication terminal, a routing control method, and a router, which control a packet transfer route.
2. Description of the Related Art
Conventionally, “IP2” (IP-based IMT Network Platform) has been proposed as a routing control method which is applied in the case where a communication terminal is moved using wireless communication (refer to, for example, T. Okagawa, M. Jo, K. Nishida, and A. Miura, “IP Packet Routing Mechanism Based on Mobility Management in IP-based IMT Network Platform”, International Conference on Intelligence in the next generation Networks (ICIN), 2003, and T. Okagawa et al., “IP Packet Routing Mechanism in IP2”, The Institute of Electronics, Information and Communication Engineers (IEICE) Technical Report, Mobile Multimedia Communications (MoMuc) 2002-63, November 2002).
In IP2, communication with a mobile communication terminal has been achieved by using two addresses: a unique host address unique to the communication terminal; and a routing host address, which is assigned to the communication terminal by an access router (edge router) and the like for routing between routers.
Specifically, as shown in FIG. 1, the access router includes: a routing cache table (RCT) 300 which is a correspondence table between a unique host address (IPha) and a routing host address (IPra); a routing table 400 having routing information which establishes correspondence between a unique host address and an output port of an access router; and a neighbor cache 500 holding information which establishes correspondence between a MAC address (layer 2 address) and an IP address (layer 3 address).
When the communication terminal starts communication through the access router, or executes handover, the access router assigns a routing host address to the communication terminal, that is, the access router establishes correspondence between the routing host address and a unique host address, and, at the same time, updates the contents of RCT 300 and the routing table 400.
FIG. 2 shows message procedure for a case where handover is executed for the communication terminal under control of the access router, that is, between wireless base stations (access points, base stations, and the like) connected to the same access router. Up on receiving a router advertisement which has been transmitted from the access router through a base station of a destination for handover, the communication terminal updates the neighbor cache created in the communication terminal, and transmits an activation message, which is a message of an upper layer above the layer 3, to the access router.
When the access router receives the activation message, the router re-assigns a routing host address to the communication terminal, and updates the contents of the routing table 400 which establishes correspondence between a unique host address and an output port.
The access router transmits an activate notification message to a routing manager, which is a routing management mechanism in a network, in order to register the correspondence between a routing host address and a unique host address, and updates the contents of RCT 300 according to a routing host address update (IPra update) message transmitted from the routing manager.
FIG. 3 shows message procedure for a case where the access router transfers a packet addressed to the communication terminal. As shown in FIG. 3, when the packet addressed to the communication terminal from the network side is received, the access router searches RCT 300 to convert the routing host address included in the received packet, to a unique host address.
The access router then selects a route to the communication terminal, using the routing table 400 based on the converted unique host address, and transmits the packet which has the converted unique host address to the communication terminal.
However, in IP2, when handover is executed between the wireless base stations connected to the same access router for the communication terminal, re-assignment of the routing host address, updating of the routing table 400, updating of RCT 300 in the access router, and updating of RCT in the routing manager are executed as described above, though the correspondence between a unique host address and a routing host address for the communication terminal has been registered in RCT 300 of the access router.
Therefore, there has been a problem in that a certain period of time is required for completion of the handover for the communication terminal even when the handover is executed between the wireless base stations connected to the same access router.
Further, since the routing information establishing correspondence between a unique host address and an output port is registered in the routing table 400, and the amount of information in the routing table 400 is increased according to an increase in the number of communication terminals communicating through the access router, there has been a problem in that the time required to search for routing information is increased, and the high-speed transfer of packets becomes difficult in IP2. Further, since the access router is required to refer to RCT 300, the routing table 400, and the neighbor cache 500 when the packets are transferred, there has also been a problem in that the high-speed transfer of packets becomes difficult.